1. Field of the Invention
A solid-liquid-vapor (S/L/V) multiple phase transformation refer to simultaneous vaporizaton and solidification operations in which a mass of liquid is simultaneously partially vaporized and partially solidified to thereby form a first vapor and a mass of solid, which may be a mass of solvent solid, a mass of solute solid or a mixed mass of solvent solid and solute solid. S/L/V multiple phase transformations are involved in such processes as (a) Vacuum Freezing Processes, (b) Primary Refrigerant Eutectic Freezing Process, (c) Distillative Freezing Processes, and (d) Vacuum Crystallization Processes. The methods and apparatuses of the present invention are used to conduct such S/L/V transformations and have the following advantages over the conventional ways of conducting the S/L/V transformations:
(a) The equipment used is simple and can be easily scaled up PA1 (b) The energy input required is greatly reduced PA1 (c) The crystals formed are larger and can be washed more easily PA1 (d) The transport of the mass of crystals is readily accomplished PA1 (e) In the eutectice freezing operation, the masses of solvent and solute crystals formed can be separated PA1 1. Rotating Sprayer Method; PA1 2. Stationary Sprayer Method; PA1 3. Spraying on Rotating Basket Method; PA1 4. Overflow Tube Method PA1 (a) a simple desublimation; PA1 (b) a mixed condensation; PA1 (c) a simple condensation PA1 (a) the first vapor is cooled to form a condensed mass which may be mass of solvent solid or a mixed mass of solid and liquid; PA1 (b) the solid part of the condensed mass is then melted so that the condensed mass becomes a liquid mass. PA1 (a) Enhanced heat and mass tranfer rates; PA1 (b) Adoptable to multistage operation PA1 (c) Low work input required in conducting the S/L/V transformation; PA1 (d) Large size crystals produced; PA1 (f) Facilitate transport of solvent and solute crystals
2. Brief Description of the Prior Art
The methods and apparatuses of the present invention are to be used in conducting solid-liquid-vapor multiple phase transformation operations (S/L/V transformation) in (a) Vacuum Freezing Processes, (b) Primary Refrigerant Eutectic Freezing Processes, (c) Distillative Freezing Processes and (d) Vacuum Crystallization Processes. An S/L/V transformation refers to simultaneous vaporization and solidification operations of a mass of liquid to thereby form a first vapor and form a mass of solid which may be a mass of solvent crystals, a mass of solute crystals, or a mixed mass of solvent and solute crystals. References to these processes are presented and the conventional methods used in accomplishing the S/L/V transformations in these processes are outlined.
2A. Vacuum Freezing Processes
A vacuum freezing processes can be used in the separation of both aqueous and non-aqueous solutions. Many vacuum freezing processes have been introduced by workers in the desalination field. Some of these processes have been tested only in bench scale units and only a few have been tested in pilot plant scale operations.
Referring to the processing of an aqueous solution by any vacuum freezing process, the aqueous solution is introduced into a chamber which is maintained at a pressure that is somewhat lower than the vapor pressure of the solution at the freezing temperature of the solution to thereby simultaneously flash vaporize water and form ice crystals. This operation is referred to as the S/L/V transformation in a vacuum freezing process. As the result of this operation, a low pressure water vapor, referred to as a first vapor, and an ice-mother liquour slurry, referred to as a first condensed mass, are formed. In the case of sea water desalination, this pressure is around 3.5 Torr. The low pressure water vapor formed has to be removed and transformed into a condensed slate; the ice crystals have to be separated from the mother liquor and the resulting purified ice has to be melted to yield fresh water. Furthermore, the heat released in transforming the vapor into a condensed state has to be utilized in supplying the heat needed in melting the ice. The processes to be described utilize different ways of vapor removal and different ways of accomplishing the heat reuse.
A sub-triple point vapor is defined as a vapor whose pressure is lower than the triple point pressure of its major component. Sub-triple point vapor desublimes to form a solvent solid upon a constant pressure cooling and the desublimation tempertaure is lower than the normal melting temperature of the solvent solid. When a sub-triple point vapor mixture is cooled, it may undergo a mixed condensation operation by partially condensing into liquid and partially condensing into solid at a temperature that is also lower then the melting temperature of the solvent solid. Therefore, the heat released in the desublimation operation or a mixed condensation operation of a sub-triple point vapor or vapor mixture cannot be used in supplying the heat needed in melting a mass of the purified solid of the major component. Conversely, a super-triple point vapor is defined as a vapor whose pressure is higher than the triple point pressure of its major component. A super-triple point vapor condenses to form a solvent liquid upon a constant pressure cooling and the condensing temperature is higher than the normal melting temperature of the solvent solid. Therefore, the heat released in condensing a super-triple point vapor can be used in supplying the heat needed in melting a mass of the purified solid of the major component.
References to the vacuum freezing processes that have been introduced are given as follows:
(1) Vacuum Freezing Vapor Compression (VFVC) Process:
This process was developed by Colt Industries and is described in the Office of Saline Water, Research and Development Report No. 295.
(2) Vacuum Freezing Vapor Absorption (VFVA) Process: This process was developed by Carrier Corporation and is described in the Office of Saline Water, Research and Development Report No. 113.
(3) Vacuum Freezing Ejector Absorption (VFEA) Process: This process was developed by Colt Industries and is described in the Office of Saline Water, Research and Development Report No. 744.
(4) Absorption Freezing Vapor Compression (AFVC) Process:
This process was introduced by Concentration Specialists, Inc., Andover, Mass. and is described in the reports submitted to the Office Water Research and Technology, Department of Interior on May 1981 and January 1982. No journal publication seems to be available.
(5) Vacuum-Freezing Vapor-Freezing (VFVF) Process: This process was introduced by Ralph E. Peck and is described in U.S. Pat. No. 3,714,791.
(6) Vacuum Freezing Multiple Phase Transformation (VFMPT) Process:
This process was introduced by Chen-Yen Cheng and Sing-Wang Cheng and is described in U.S. Pat. Nos. 4,420,318 and 4,505,728.
(7) Vacuum Freezing Solid Condensation (VFSC) Process: This process was introduced by H. M. Curran and C. P. Howard of Catholic University of America and is described in the Office of Saline Water, Research and Development Report No. 511.
(8) Vacuum Freezing High-Pressure Ice-Melting (VFPIM) Process:
This process was introduced by Chen-Yen Cheng and Sing-Wang Cheng and is described in U.S. Pat. No. 4,236,382.
Methods used in these processes to accomplish the S/L/V transformations are as followed:
The VFVC, VFEA, AFVC, VFMPT and VFPIM Processes use the rotating sprayer method; the VFVF process used the stationary sprayer method; the VFSC Process uses the spraying on rotating basket method; the VFVA Process uses the overflow tube method.
The rotating sprayer method used in VFVC Process uses an agitator that consists of four pipes arranged in a scoop-type fashion to scoop solution from a liquid pool and throw it on the freezer walls, and at the same time agitate the pool. In the bottom of the freezer are baffles that dampen the vortex formed by the rotating agitator. Water vaporizes from the surfaces of the spray and the liquid film on the wall to cause simultaneous freezing. The methods used in VFEA, AFVC, VFMPT and VFPIM Processes are similar.
The stationary sprayer method is used in the VFVF Process. In the process, a batch evaporative desalination method and apparatus having a pair of similar systems for substantially continuous output are described. Each system has three evacuated chambers in vapor communication with each other. In the first chamber, precooled seawater is sprayed for partial vaporization and consequent formation of ice crystals as latent heat is removed from the seawater. Ice crystals are permitted to accumulate in the first chamber and water vapor flows to a second chamber in which refrigeration coils, preferably cooled by cold natural gas, maintained at a temperature below the triple point on that ice condenses thereon. After a selected interval, spraying of precooled seawater into the first chamber and refrigeration in the second chamber are stopped. Warmer seawater is them sprayed into a third chamber also maintained low pressure so that a portion of the water vaporizes but without formation of ice in the brine. The water vapor flows to the first and second chambers and condenses on the ice therein to transfer intent heat for melting the ice. Fresh water is withdrawn from the second chamber. Fresh water in the first chamber percolates through the ice crystals for washing and when residual brine is removed, fresh water is withdrawn from this chamber also.
The spraying on rotating basket method is used in the VFSC Process. The process is a batch evaporative freezing process in which saline water is sprayed into a rotating cylindrical basket at a pressure below the triple point pressure. Continuous removal of vapor results in the formation of an annular ice-brine semi-solid layer on the lateral surface of the basket. The brine is removed by washing and the residual ice is melted. The optimum design requires that the freezing, washing and melting operations be of equal duration. Therefore, the optimum plant consists of three modules, in each of which the freezing, washing and melting operations occur in succession and out-of-phase with the other two. The heat removed by evaporative freezing in one module is used to melt the washed ice in another module.
The overflow tube method is used in the VFVA Process. In this process, the S/L/V transformation operation is conducted by pumping brine through standing tubes and allow the brine to overflow along the outer walls of the standing tubes. Simultaneous vaporization and freezing take place from the overflowing liquid film.
2B. Primary Refrigerant Eutectic Freezing (PREUF) Process
The Primary Refrigerant Eutectic Freezing (PREUF) Process has been introduced by Chen-Yen Cheng and Sing-Wang Cheng and is described in U.S. Pat. No. 4,654,064. The process is used to separate mixtures containing at least one volatile component and two or more crystal-forming components. Heat is removed from a eutectic mixture at near its eutectic temperature by inducing vaporization of a portion of the eutectic mixture at its eutectic temperature. The vapor is liquefied by a two-step process--(a) mixed condensation/desublimation and (b) desublimate-melting. Co-crystallizatiuon of different components in the same zone of the freezer or selective crystallization of different components in different sub-zones of the freezer are possible with several different flow schematics possible. Separation of crystals of different components formed in co-crystallization in the same liquid pool followed by separation of individual component crystals from adhering liquids give purified products. Separation of crystals of different components is not required where selective crystallization is effective.
Two methods of conduction S/L/V transformations are described in the patent. The first method is to apply a film of liquid on a vertical plate and allow the liquid to partially vaporized and solidify; the second method is to add a mass of liquid on a rotating horizontal tray to thereby partially vaporize and solidify the liquid.
2C. Wet and Dry Distillative Freezing (DF) Process
Wet and Dry Distillative Freezing Process has been introduced by Chen-Yen Cheng and Sing-Wang Cheng and is described in U.S. Pat. No. 4,578,093. A wet and dry distillative freezing process comprises (a) a first step of transforming a liquid feed mixture into a first solid-liquid mixture, denoted as K.sub.1 mixture, by either a conventional freezing operation or a wet distillative freezing operation, (b) a second step of washing the K.sub.1 mixture with a wash liquid to thereby form a second solid-liquid mixture, denoted as K.sub.2 mixture, and an impure liquid L.sub.2, and (c) a third step of subjecting the K.sub.2 mixture to a dry distillative freezing opertion to thereby form a mass of refined solid phase, denoted as S.sub.3, and a low pressure vapor V.sub.1. Various wash liquids may be used in the crystal washing step. It is important to note that the wash liquid used does not have to be a pure liquid but may contain some voltaile impurities. This is so, because the volatile impurities in the wash liquid will be taken up in the K.sub.2 mixture and will be removed in the dry distillative freezing step. Convenient wash liquids to use are (a) a mass of the feed liquid, (b) a mass of the condensate liquid, and (c) a part of the product liquid. One may also use a solution containing the crystallizing component and a selected volatile component as a wash liquid. In this process, the wet distillative freezing operation, which is an S/L/V transformation is accomplished by using a set of rotating disks rotated by a shaft. These disks are partially submerged into liquid and are scraped by stationary blades.
2D. Vacuum Crystallization
Vacuum crystallization is a very well known unit operation and is described in most books on unit operations. An extensive description is available in Kirk and Othmer: "Encyclopedia of Chemical Technology." In this process, the S/L/V transformation causes formation of a vapor stream and a mass of solute crystals. Solution is heated mostly in a long vertical tube heat exchanger and the heated solution is flash vaporized in a vacuum chamber connected to the top of the vertical tubes.